PBX with T1/E1 Multiplexer ?

Dear Canopy Experts,

I want to build PBX connection between 2 building so i can use extention line for call each 2 building.
the layout like this :

PBX 2---- Multiplexer — BH ------- BH ------ Multiplexer ----PBX 1

Can i get step by step to set up that?
n if i use different vendor PBX, Can talk each other? I mean if i call ext from PBX 2 so i can talk with ext at PBX 1 ?

Thanks for advice